Transcoders and mixers for voice-over-IP conferencing

ABSTRACT

Transcoders and mixers having reduced algorithmic delay and processing complexity. An improved mixer for signals having encoded speech parameters wherein the parameters obtained through decoding are used by a parameter estimator to improve the encoding by providing a parameter estimate for the mixed signal. In the case of pitch parameters, the mixer uses the principle of strong-pitch-domination. The mixing of wideband signals is simplified by performing mixing of individual lower and upper sub-bands. A transcoder and a mixer that converts a wideband signal into a narrowband signal relies upon high frequency suppression. A transcoder and a mixer that converts a narrowband signal into a wideband signal relies upon filter combination.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. provisional patent applicationSer. No. 60/488,254, filed Jul. 18, 2003, the entire contents of whichis hereby incorporated by reference.

FIELD OF THE INVENTION

This invention relates to signal processing in packet-based networksand, in particular, to transcoders and mixers for use in packet-basednetworks.

BACKGROUND OF THE INVENTION

Digital packet-based networks, like the Internet, are increasingly beingused to transmit voice signals. Given the asynchronous nature ofpacket-based networks, any extra complexity or delay in the network canpresent problems when the voice signal is reconstructed from itspacketized form at the receiving end.

Voice-over-Internet-Protocol (VoIP) technology attempts to provide forvoice communication over the Internet through the use of variouscommunications protocols by which the voice signals can be encoded fortransmission and decoded when received. In some cases, participants in aconversation will be using different protocols. Accordingly, it isnecessary to convert a VoIP signal encoded in one protocol to a VoIPsignal encoded using another protocol.

In a multi-speaker environment, the ability to mix signals from variousparticipants in a mixer is important to providing a Quality of Service(QoS) comparable with traditional dedicated public-switched telephonenetwork (PSTN) teleconferencing. Again, if participants are usingdifferent codecs, then the mixer must accommodate these differences andconsolidate the signals into one mixed signal using a selectedcommunications protocol.

Typical VoIP communications protocols include narrowband protocolsG.711, G.729, and G.729(A), and wideband protocols 722 and 722.2.

The various communications protocols are typically applied to signalsusing codecs (encoders/decoders). Codecs are signal processing devices,usually implemented on a digital signal processor. They typicallyoperate on a frame by frame basis, often with a buffer of frames for‘lookahead’ purposes and/or to reduce jitter. This tends to introducedelay and complexity such that, in a mixer, the process of decoding asignal, mixing it with another signal, and encoding the mixed signal canresult in problems, including packet loss, jitter, and end-to-end delay.All of these problems lead to difficulties in obtaining satisfactory QoSfor VoIP.

Accordingly, there remains a need for VoIP technology having an improvedQoS through reduced complexity and delay in transcoding and mixing.

SUMMARY OF THE INVENTION

The present invention provides improved VoIP technology through reducedalgorithmic and processing complexity and delay.

In one aspect, the present invention provides a VoIP mixer for mixing afirst input signal with a second input signal, the first and secondinput signals being signals having encoded therein a first and secondcorrelation parameter, respectively. The VoIP mixer includes (a) a firstdecoder for receiving the first input signal and outputting a firstdecoded signal, the first decoder extracting the first correlationparameter from the first input signal, (b) a second decoder forreceiving the second input signal and outputting a second decodedsignal, the second decoder extracting the second correlation parameterfrom the second input signal, (c) a mixer coupled to the first andsecond decoders, the mixer receiving the first and second decodedsignals and producing a mixed signal, (d) a parameter estimator coupledto the first and second decoders, the parameter estimator receiving thefirst and second correlation parameters and outputting an open loopparameter estimate, and (e) an encoder coupled to the mixer and theparameter estimator, the encoder receiving the mixed signal and the openloop parameter estimate and outputting an encoded signal, wherein theencoder includes a closed-loop analyzer for creating the encoded signaland wherein the closed-loop analyzer employs the open loop parameterestimate.

In a further aspect, the present invention provides a method for mixinga first input signal with a second input signal in a VoIP system, thefirst and second input signals comprising signals having encoded thereina first and second correlation parameter, respectively. The methodincludes the steps of, in a decoder, extracting the first and secondcorrelation parameters from the first and second input signals, decodingthe first and second input signals and outputting a first decoded signaland a second decoded signal, mixing the first and second decoded signalsto produce a mixed signal, determining an open loop parameter estimatebased upon said extracted first and second correlation parameters, andencoding the mixed signal, wherein the step of encoding includesperforming a closed loop analysis to obtain a mixed signal correlationparameter for use in encoding the mixed signal, and wherein the closedloop analysis employs the open loop parameter estimate.

In another aspect, the present invention provides a VoIP mixer formixing a first input signal with a second input signal, the first andsecond input signals comprising wideband signals each having an uppersub-band and a lower sub-band. The VoIP mixer includes a first widebanddecoder for receiving the first input signal and outputting a firstlower sub-band decoded signal and a first upper sub-band decoded signal,a second wideband decoder for receiving the second input signal andoutputting a second lower sub-band decoded signal and a second uppersub-band decoded signal, a lower sub-band mixer for receiving the firstand second lower sub-band decoded signals and producing a mixed lowersub-band signal, a upper sub-band mixer receiving the first and secondupper sub-band decoded signals and producing a mixed upper sub-bandsignal, and a wideband encoder for receiving the mixed lower and uppersub-band signals and outputting an encoded mixed signal.

In a further aspect, the present invention provides method for mixing afirst input signal with a second input signal in a VoIP system, thefirst and second input signals comprising wideband signals each havingan upper sub-band and a lower sub-band. The method includes the steps of(a) decoding the first input signal to produce a first lower sub-banddecoded signal and a first upper sub-band decoded signal, (b) decodingthe second input signal to produce a second lower sub-band decodedsignal and a second upper sub-band decoded signal, (c) mixing the firstand second lower sub-band decoded signals to produce a mixed lowersub-band signal, (d) mixing the first and second upper sub-band decodedsignals to produce a mixed upper sub-band signal, and (e) encoding themixed lower and upper sub-band signals to produce an encoded mixedwideband signal.

In another aspect, the present invention provides a transcoder forconverting a first input signal to an output signal, the first inputsignal comprising a wideband signal having an upper sub-band and a lowersub-band, and the output signal being a narrowband signal. Thetranscoder includes a lower sub-band decoder having an input forreceiving the input signal and an output for providing a decoded lowersub-band signal, a low pass filter having an input for receiving thedecoded lower sub-band signal and an output for providing a filteredlower sub-band signal, and a narrowband encoder for encoding thefiltered lower sub-band signal to produce the output signal. In afurther aspect, the present invention provides a VoIP mixer includingsuch a transcoder.

In a further aspect, the present invention provides a method forconverting a first input signal to an output signal in a VoIP system,the first input signal comprising a wideband signal having an uppersub-band and a lower sub-band, and the output signal comprising anarrowband signal. The method includes the steps of decoding the lowersub-band of the input signal to produce a lower sub-band signal, lowpass filtering the lower sub-band signal to produce a filtered lowersub-band signal, and encoding the filtered lower sub-band signal toproduce the output signal. In a further aspect, the present inventionprovides a method of mixing that includes such method steps.

In yet another aspect, the present invention provides a transcoder forconverting a first input signal to an output signal, the first inputsignal comprising a narrowband signal and the output signal comprising awideband signal. The transcoder includes a narrowband decoder forreceiving the first input signal and outputting a decoded signal, afirst filter for receiving the decoded signal and outputting a firstlower sub-band signal, the first filter, the first filter having atransfer characteristic for introducing a first artificial aliasing intothe decoded signal to produce the first lower sub-band signal, a secondfilter for receiving the decoded signal and outputting a first uppersub-band signal, the second filter having a transfer characteristic forintroducing a second artificial aliasing into the decoded signal toproduce the first upper sub-band signal, and a wideband encoder forreceiving the first lower sub-band signal and the first upper sub-bandsignal, encoding the sub-band signals, and producing the output signal.In a further aspect, the present invention provides a VoIP mixerincluding such a transcoder.

In yet a further aspect, the present invention provides method forconverting a first input signal to an output signal in a VoIP system,the first input signal comprising a narrowband signal and the outputsignal comprising a wideband signal. The method includes the steps ofnarrowband decoding the first input signal to produce a decoded signal,filtering the decoded signal to produce a first lower sub-band signal,wherein said filtering includes introducing a first artificial aliasinginto the decoded signal to produce the first lower sub-band signal,filtering the decoded signal to produce a first upper sub-band signal,wherein said filtering includes introducing a second artificial aliasinginto the decoded signal to produce the first upper sub-band signal, andwideband encoding the first lower sub-band signal and the first uppersub-band signal to produce the output signal. In a further aspect, thepresent invention provides a method of mixing that includes such methodsteps.

Other aspects and features of the present invention will become apparentto those ordinarily skilled in the art upon review of the followingdescription of specific embodiments of the invention in conjunction withthe accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will now be made, by way of example, to the accompanyingdrawings which show an embodiment of the present invention, and inwhich:

FIG. 1 shows, in block diagram form, an embodiment of a VoIP mixer formixing signals having encoded speech parameters;

FIG. 2 shows, in flowchart form, a method of mixing signals havingencoded speech parameters in an VoIP system;

FIG. 3 shows, in block diagram form, an embodiment of a VoIP mixer formixing signals having upper and lower sub-bands;

FIG. 4 shows, in flowchart form, a method for mixing signals havingupper and lower sub-bands in a VoIP system;

FIG. 5 shows a block diagram of a VoIP transcoder for converting awideband signal to a narrowband signal;

FIG. 6 shows, in flowchart form, a method of transcoding a widebandsignal into a narrowband signal in a VoIP system;

FIG. 7 shows a block diagram of a VoIP mixer for mixing an inputnarrowband signal with an input wideband signal to produce an outputnarrowband signal;

FIG. 8 shows, in flowchart form, a method for mixing an input narrowbandsignal with an input wideband signal to produce an output narrowbandsignal in a VoIP system;

FIG. 9 shows a block diagram of a VoIP transcoder for converting anarrowband signal to a wideband signal;

FIG. 10 shows graphs of the frequency responses of first and secondcombined filters in a filter-combination VoIP transcoder;

FIG. 11 shows, in flowchart form, a method for transcoding a narrowbandsignal into a wideband signal in a VoIP system;

FIG. 12 shows a block diagram of a VoIP mixer for mixing an inputnarrowband signal with an input wideband signal to produce an outputwideband signal; and

FIG. 13 shows, in flowchart form, a method for mixing an inputnarrowband signal with an input wideband signal to produce an outputwideband signal, in a VoIP system.

Similar reference numerals are used in different figures to denotesimilar components.

DESCRIPTION OF SPECIFIC EMBODIMENTS

The following detailed description of specific embodiments of thepresent invention does not limit the implementation of the invention toany particular programming language or signal processing architecture.In one embodiment, the present invention is implemented, at leastpartly, using a digital signal processor. It will be understood that thepresent invention may be implemented using other architectures,including a microcontroller, a microprocessor, discrete components, orcombinations thereof. Any limitations presented herein as a result of aparticular type of architecture or programming language are not intendedas limitations of the present invention.

Reference is first made to FIG. 1, which shows, in block diagram form,an embodiment of a VoIP mixer 10 for mixing signals having encodedspeech parameters.

The VoIP mixer 10 includes a first decoder 12, a second decoder 14, amixer 16, and an encoder 18. The first decoder 12 receives a first inputsignal 20 and the second decoder 14 receives a second input signal 22.The first and second input signals 20, 22 are encoded using acommunications protocol that encodes various speech parameters withinthe signal. These parameters may include parameters reflecting short-and/or long-term correlations in the speech signal and other parametersfor refining the signal.

In one embodiment, the first and second input signals 20, 22 arenarrowband signals encoded using the G.729(A) protocol. The G.729(A)protocol employs conjugate-structure algebraic-code-excitedlinear-predictive (CS-ACELP) coding. Each frame of speech in a G.729(A)signal is 10 milliseconds in duration and is based upon 80 samples at asampling rate of 8 kHz. For each frame, the encoder extracts theparameters of the CELP model and encodes these parameters within thesignal. The parameters include line spectrum pairs (LSP) for short termcorrelations in the signal, adaptive codebook parameters such as pitchparameters for long-term correlations in the signal, and fixed-codebookparameters to refine the excitation signal.

A G.729(A) encoder performs LP analysis and an open-loop pitch searchonce per frame. The open-loop pitch search analyzes the perceptualweighted signal to find a rough open-loop pitch delay T_(op). Theencoder then performs a closed-loop pitch search to find the optimalpitch delay for the current subframe. Through using the estimate ofopen-loop pitch delay T_(op), the complexity and delay of theclosed-loop pitch search is reduced by limiting the search range aroundT_(op). An algebraic codebook search, adaptive-codebook gain andalgebraic-codebook gain quantizations, and a memory update are performedonce per subframe.

A G.729(A) decoder receives an input signal and decodes the parametersencoded in the signal. The parameters, such as the pitch parameters,gains, LSPs, and fixed codebook vectors, are then used to obtain theexcitation and synthesis filter parameters. The speech signal is thenreconstructed by filtering this excitation through the short termsynthesis filter and further enhanced by post filtering.

Referring back to FIG. 1, the first and second decoders 12, 14 in theVoIP mixer 10 each include a parameter decoder 24, 26 that extractsspeech parameters 28, 30 from the input signal 20, 22. The speechparameters 28, 30 are employed in creating a synthesis filter 32, 34,which together with a post filter 36, 38 produces first and seconddecoded PCM signals 40, 42, respectively.

The first and second PCM signals 40, 42 obtained from the first andsecond decoders 12, 14 are mixed together in the mixer 16, which outputsa mixed signal 44. This mixed signal 44 is then converted to an encodedmixed signal 46 by the encoder 18.

The VoIP mixer 10 further includes a parameter estimator 50. Theparameter estimator 50 receives at least one pair of speech parameters28, 30 from the first and second parameter decoders 24, 26. Based uponthe at least one pair of speech parameters 28, 30, the parameterestimator 50 produces a parameter estimate 56 that is input to theencoder 18 for use in encoding the mixed signal 44. By having theencoder 18 utilize an estimated parameter derived from one or more ofthe speech parameters 28, 30 already obtained during the decodingprocess by the decoders 12, 14, the delay and complexity of the encoder18 can be reduced and the overall speed of the mixing is enhanced.

In one embodiment, the parameter estimator 50 is a pitch estimator thatreceives first and second pitch parameters 52, 54 from the decoders 12,14. The first and second pitch parameters 52, 54 each include values forpitch delay, pitch gain and frame energy. The parameter estimate 56produced by the pitch estimator 50 is an estimated pitch delay.

In one embodiment, the pitch estimator 50 determines the estimated pitchdelay by comparing the pitch energies of the input first and secondpitch parameters 52, 54 and choosing the one with the strongest pitchenergy. Accordingly, the pitch estimator 50 utilizes a principle ofstrong-pitch-domination, i.e. that the input signal with the strongestpitch energy will dominate the pitch characteristics of the mixedsignal. In one embodiment, the pitch energies are assessed byconsidering the signal energy, the pitch gain normalized by the signalenergy, and the pitch variation. By default, it is assumed that thepitch delay should not vary greatly for voiced speech. Accordingly, ifthe pitch delay differs from frame to frame by more than 20, a smallerweight is given to the case on the basis that the frame relates to anunvoiced or transient period of the speech.

The estimated pitch delay is used by the encoder 18 instead of anopen-loop pitch delay search. The elimination of the open-loop pitchdelay search reduces the complexity of the mixer by more than 7%.

Referring still to FIG. 1, the encoder 18 includes a pre-processingmodule 58 and a line spectrum pairs (LSP) analysis module 60. Theencoder 18 also includes a codebook search module 62, which includes aclosed-loop pitch search component 64 and an adaptive fixed codebooksearch component 66. The closed-loop pitch search component 64 receivesthe estimated pitch delay from the pitch estimator 50.

Reference is now made to FIG. 2, which shows in flowchart form a method100 of mixing signals having encoded speech parameters in an VoIPsystem.

The method 100 begins in step 102 with the receipt of first and secondsignals. The first and second signals are encoded using a communicationsprotocol that encodes various speech parameters within the signals.These parameters may include parameters reflecting short- and/orlong-term correlations in the speech signal and other parameters forrefining the signal. One such communications protocol is the G.729(A)protocol. The G.729(A) protocol encodes various speech parameters in anencoded signal, including pitch parameters.

In step 104, the signals are each decoded to obtain the encoded speechparameters and in step 106 the decoding of each signal is completed (inpart, using the decoded speech parameters in accordance with thecommunications protocol) to obtain first and second decoded PCM signals.

Having obtained speech parameters for the first and second signalsduring step 104, the speech parameters are then used in step 108 togenerate a parameter estimate.

The decoded PCM signals are mixed in a mixer in step 110 to create amixed signal, which is then encoded by an encoder in step 112. Theencoder uses the parameter estimate generated in step 108 to assist inencoding the mixed signal in step 112. Finally, in step 114, an encodedmixed signal is output.

It will be understood by those of ordinary skill in the art that thepresent invention is not limited to strong-pitch-domination. Variousother parameters identified in the decoding process may be provided tothe parameter estimator 50 (FIG. 1) in order to produce a parameterestimate 56 for use by the encoder 18, including the linear spectrumpairs parameters and the algebraic codebook parameters.

It will also be understood by those of ordinary skill in the art thatthe present invention is not limited to narrowband G.729(A) protocolmixers, but extends to mixers for any input signal encoded using aprotocol that encodes speech parameters that are extracted for thedecoding process and are calculated during the encoding process. Withoutlimiting the scope of the present invention, other such protocolsinclude G.722.2, which also employs the ACELP model.

Reference is now made to FIG. 3, which shows, in block diagram form, anembodiment of a VoIP mixer 200 for mixing signals having upper and lowersub-bands.

In a conventional mixer for VoIP signals having upper and lowersub-bands, the first and second signals are individually decodedaccording to the relevant communications protocol. For each signal thistypically involves demultiplexing the signal into its respective upperand lower sub-bands and decoding each sub-band according to thecommunications protocol. The decoded upper and lower sub-bands are thencombined in a receive quadrature mirror filter (QMF) to create a decodedwideband signal. The first and second decoded wideband signals thuscreated are then combined in a mixer. The output of the mixer is a mixedwideband signal.

Following the mixer, the mixed wideband signal is then passed through atransmit QMF to separate the mixed wideband signal into upper and lowersub-bands. These upper and lower sub-bands are then encoded according tothe communications protocol and recombined in a multiplexer fortransmission within the VoIP system.

The VoIP mixer 200 shown in FIG. 3 eliminates the need for QMFs byperforming sub-band mixing. The VoIP mixer 200 includes first and seconddemultiplexers 202, 204, first and second lower sub-band decoders 206,208, first and second upper sub-band decoders 210, 212, first and secondmixers 214, 216, a lower sub-band encoder 218, an upper sub-band encoder220, and a multiplexer 222.

The VoIP mixer 200 receives a first input signal 224 at the firstdemultiplexer 202 and a second input signal 226 at the seconddemultiplexer 204. The first and second input signals 224, 226 arewideband signals having upper and lower sub-bands.

In one embodiment, the first and second input signals 224, 226 areencoded using the G.722 protocol. G.722 uses sub-band adaptivedifferential pulse code modulation (SB-ADPCM) within a bit rate of 64kbit/s. The frequency band is split into higher and lower sub-bands andeach sub-band is encoded using ADPCM technology. Because of theperceptual importance of the lower sub-band, G.722 allocates more bitsto the lower sub-band than the higher sub-band, resulting in a 48 kbit/slower sub-band stream and a 16 kbit/s higher sub-band stream. Thesestreams are then combined into a 64 kbit/s stream using a multiplexer toproduce an encoded G.722 signal.

In the VoIP mixer 200, the demultiplexers 202, 204 output first lowerand upper sub-band streams and second lower and upper sub-band streams,which are input to the first lower and upper sub-band decoders 206, 210,and the second lower and upper sub-band decoders 208, 212, respectively.The first lower sub-band decoder 206 produces a decoded first lowersub-band signal 228. The first upper sub-band decoder 210 produces adecoded first upper sub-band signal 230. Similarly, the second lowersub-band decoder 208 produces a decoded second lower sub-band signal 232and the second upper sub-band decoder 212 produces a decoded secondupper sub-band signal 234.

The two decoded lower sub-band signals 228, 232 are mixed together inthe first mixer 214 to produce a lower sub-band mixed signal 236. Thetwo decoded upper sub-band signals 230, 234 are mixed together in thesecond mixer 216 to produce an upper sub-band mixed signal 238.

The lower and upper sub-band mixed signals 236, 238 are then encoded bythe lower sub-band encoder 218 and the upper sub-band encoder 220,respectively, and the outputs are multiplexed by the multiplexer 222 toproduce a mixed wideband output signal 240.

By mixing the two signals at the sub-band level, the VoIP mixer 200reduces the hardware complexity by eliminating the need for quadraturemirror filters. The reduction in additional filtering also improvessignal quality and reduces algorithmic delay. The overall reduction inmixer complexity is about 37% for the multiplications and about 45% forthe additions.

Reference is now made to FIG. 4, which shows in flowchart form a method250 for mixing signals having upper and lower sub-bands in a VoIPsystem.

The method 250 begins in step 252 with receiving a first and a secondsignal. The first and second signals are wideband signals having upperand lower sub-bands. An example of a communications protocol thatresults in such signals is the G.722 protocol.

In step 254, the first and second signals are separated into theirrespective upper and lower sub-bands. In one embodiment, this step isperformed by a demultiplexer. Then, in step 256, each of the upper andlower sub-band signals resulting from step 254 are decoded according tothe relevant communications protocol used to encode them, so as toproduce decoded upper and lower sub-band signals. For example, with theG.722 protocol, ADPCM decoding is utilized to recover the decodedsub-band signals.

In step 258, the first and second decoded lower sub-band signals aremixed in a mixer, and in step 260, the first and second decoded uppersub-band signals are mixed in a mixer. The output of each of themixers—lower and upper mixed sub-band signals, respectively—are thenencoded in steps 262 and 264, in accordance with the relevantcommunications protocol. The encoded upper and lower mixed sub-bandsignals are then combined in step 266 to produce the mixed widebandoutput signal.

Reference is now made to FIG. 5, which shows a block diagram of a VoIPtranscoder 300 for converting a wideband signal to a narrowband signal.

In a conventional VoIP transcoder for converting a wideband signalhaving upper and lower sub-bands to a narrowband signal, the widebandsignal is decoded according to the relevant communications protocol andthen downsampled and encoded as a narrowband signal, i.e. converted intoA- or μ-law PCM. The decoding of the wideband signal typically involvesdemultiplexing the signal into its respective upper and lower sub-bandsand decoding each sub-band according to the communications protocol andthen recombining the signals using a receive QMF. The downsampling isthen performed by low pass filtering the decoded wideband signal at 16kHz and deleting alternate samples.

The VoIP transcoder 300 shown in FIG. 5 eliminates the need for QMFs byusing only the lower sub-band of the input signal. The VoIP transcoder300 includes a lower sub-band decoder 302, a low pass filter 304, and anencoder, which in one embodiment is an A- or μ-law converter 306.

The VoIP transcoder 300 receives an input wideband signal 308 at thelower sub-band decoder 302. The input wideband signal 308 is a widebandsignal having upper and lower sub-bands. In one embodiment, the inputwideband signal 308 is encoded using the G.722 protocol.

The lower sub-band decoder 302 includes a demultiplexer for separatingthe upper and lower sub-bands within the input wideband signal 308. Italso includes a decoder for the lower sub-band signal that decodes thelower sub-band in accordance with the relevant communications protocol,such as the ADPCM decoding required by G.722. The output of the lowersub-band decoder 302 is a decoded lower sub-band signal 310.

The lower sub-band signal 310 is filtered by the low pass filter 304,which has a cutoff at 8 kHz. The low pass filter 304 removes highfrequency aliasing present in the lower sub-band signal 310 as a resultof the quadrature mirror filter present in the encoding process. The lowpass filter 304 produces a filtered signal 312 that is then converted toan encoded narrowband signal 314 by the A- or μ-law converter 306. Inone embodiment, the encoded narrowband signal 314 is a G.711 signal,encoded according to the G.711 communications protocol by the A- orμ-law converter 306.

The signal processing performed by the transcoder 300 to convert awideband signal to a narrowband signal may be referred to as highfrequency suppression.

A method 350 of transcoding a wideband signal into a narrowband signalin a VoIP system is shown in flowchart form in FIG. 6. The method 350uses high frequency suppression.

The method 350 begins in step 352 with receipt of the input widebandsignal. Then, in step 354, the lower sub-band portion of the widebandsignal is decoded according to the relevant communications protocol,which, in one embodiment, comprises the G.722 protocol. As mentionedabove, step 354 may include demultiplexing the input wideband signalinto its upper and lower sub-bands and decoding the lower sub-band toproduce a decoded lower sub-band signal.

In step 356, the lower sub-band signal is filtered by a low pass filterto remove aliasing from the upper sub-band signal that was introducedwhen the wideband signal was originally encoded. The filtered lowersub-band signal is then converted to A- or μ-law PCM by a converter toproduce the output narrowband signal.

The high frequency suppression used to convert a wideband signal into anarrowband signal may also be employed in mixing a narrowband signal anda wideband signal to produce a narrowband output signal.

Reference is now made to FIG. 7, which shows a block diagram of a VoIPmixer 400 for mixing an input narrowband signal with an input widebandsignal to produce an output narrowband signal.

In a conventional hybrid mixer for receiving a narrowband signal and awideband signal and outputting a narrowband mixed signal, the widebandsignal is converted to a narrowband signal and is mixed with the inputnarrowband signal. As with the high frequency suppression transcoder 300described above in connection with FIG. 5, the hybrid mixer may beimproved by working only with the lower sub-band component of the inputwideband signal.

Like the high frequency suppression transcoder 300 (FIG. 5), the VoIPmixer 400 includes a lower sub-band decoder 404, a low pass filter 406,and a narrowband encoder, which in one embodiment is an A- or μ-lawconverter. The VoIP mixer 400 further includes a narrowband decoder 402and a mixer 408.

The VoIP mixer 400 receives an input narrowband signal 412 and processesthe input narrowband signal 412 using the narrowband decoder 402 toproduce a decoded narrowband signal 418. In one embodiment, the inputnarrowband signal 412 is encoded according to the G.711 communicationsprotocol.

An input wideband signal 414 is received and processed by the lowersub-band decoder 404, which produces a decoded lower sub-band signal. Aswith the lower sub-band decoder 302 (FIG. 5) in the VoIP transcoder 300(FIG. 5), the lower sub-band decoder 404 in the VoIP mixer 400 includesa demultiplexer for separating the upper and lower sub-bands within theinput wideband signal 414 and a decoder for the lower sub-band signalthat decodes the lower sub-band in accordance with the relevantcommunications protocol, such as the ADPCM decoding required by G.722.

The decoded lower sub-band signal is then filtered by the low passfilter 406, which removes high frequency aliasing present in the lowersub-band signal as a result of the quadrature mirror filtering presentin the encoding process. The low pass filter 406 has a cutoff frequencyof 8 kHz and is similar to the low pass filter 304 (FIG. 5) describedwith respect to the VoIP transcoder 300 (FIG. 5). The low pass filter406 outputs a filtered decoded narrowband signal 420.

The filtered decoded narrowband signal 420 and the decoded narrowbandsignal 418 are mixed by the mixer 408, and the output mixed narrowbandsignal is converted into an encoded narrowband signal 416 by the A- orμ-law converter 410.

It will be understood that the narrowband encoders discussed above—i.e.the A- or μ-law converters 306 (FIG. 5) and 410 (FIG. 7) for G.711encoding—and the corresponding narrowband decoder 402, are not limitedto G.711 encoding/decoding. Other narrowband communications protocolsmay be used, for example G.729, G.729(A), and others.

By using high frequency suppression in a hybrid bandwidth to narrowbandmixer, the overall complexity of the mixer is reduced by abouttwo-thirds and the overall delay is improved.

Reference is now made to FIG. 8, which shows, in flowchart form, amethod 450 for mixing an input narrowband signal with an input widebandsignal to produce an output narrowband signal in a VoIP system.

The method 450 begins in step 452 when the input signals are received.The input signals include a narrowband signal and a wideband signal. Thenarrowband signal is encoded according to a narrowband communicationsprotocol, such as G.711, G.729, or others. The wideband signal isencoded according to a wideband communications protocol, such as G.722,or others.

In step 454, the input narrowband signal is decoded by a narrowbanddecoder in accordance with the relevant communications protocol,producing a decoded narrowband signal.

In step 456, the input wideband signal is split into its lower sub-bandand upper sub-band, and the lower sub-band is decoded by a lowersub-band decoder in accordance with the relevant communicationsprotocol. For example, in one embodiment, the wideband signal is encodedusing the G.722 protocol, so the lower sub-band is decoded with aSB-ACPCM decoder configured in accordance with G.722.

The decoded lower sub-band signal resulting from step 456 is thenfiltered by a low pass filter in step 458. The low pass filter has acutoff frequency at 8 kHz and results in a filtered lower sub-bandsignal.

In step 460, the filtered lower sub-band signal is mixed with thedecoded narrowband signal in a mixer to produce a mixed signal. Themixed signal is then encoded in step 462 by a narrowband encoder inaccordance with a narrowband communications protocol, such as G.711.

Reference is now made to FIG. 9, which shows a block diagram of a VoIPtranscoder 500 for converting a narrowband signal to a wideband signal.The VoIP transcoder 500 includes a first combined filter 502, a secondcombined filter 504, a lower sub-band encoder 508, an upper sub-bandencoder 510, a narrowband decoder 506, and an adder 518.

In a conventional VoIP transcoder for converting a narrowband signal toa wideband signal, where the wideband signal has upper and lowersub-bands, the narrowband signal is decoded according to the relevantcommunications protocol and then up-sampled, lowpass filtered andencoded as a wideband signal. The encoding as a wideband signal includespassing the signal through a transmit QMF to create upper and lowersub-bands and then encoding the lower sub-band with a lower sub-bandencoder and encoding the upper sub-band with an upper sub-band encoder.The two encoded sub-bands are then combined into an output widebandsignal.

The transmit QMF can be considered a two-channel polyphase filter. Infact, the transmit QMF can be modeled as a low pass filter anddown-sampler in parallel with a high pass filter and down-sampler. Thefilters of this model can be combined with the low pass filter appearingahead of the transmit QMF to model the conventional VoIP transcoder asan up-sampler followed by a first combination filter and down-sampler inparallel with a second combination filter and down-sampler. Using thepoly-phase structure again, the up-sampler and down-samplers may beeliminated by changing the first combination filter to the firstcombined filter 502 that includes the even coefficients of the firstcombination filter and by changing the second combination filter to thesecond combined filter 504 that includes the even coefficients of thesecond combination filter.

Reference is now made to FIG. 10, which shows graphs of the frequencyresponses of the first and second combined filters 502, 504 (FIG. 9),according to one embodiment of the present invention. The graphs includea first graph 550 of the magnitude versus frequency for the firstcombined filter 502, a second graph 552 of the phase versus frequencyfor the first combined filter 502, a third graph 554 of the magnitudeversus frequency for the second combined filter 504, and a fourth graph556 of the phase versus frequency for the second combined filter 504.

The frequency response of the two combined filters 502 and 504 isdesigned to mimic the aliasing found in lower sub-band and uppersub-band signals as a result of quadrature mirror filtering, so as to“trick” the lower and upper sub-band encoders and any subsequentwideband decoder into believing it has received a wideband encodedsignal. This process may be referred to as filter-combination.

In one embodiment, the coefficients of the first and second combinedfilters 502 and 504 are, respectively:

-   -   H₅₀₂=[0.0000 −0.0001 0.0003 −0.0010 0.0022 −0.0053 0.0437 0.0315        −0.0655 0.4274 0.1687 −0.0795 0.0831 −0.0004 0.0031 −0.0015        0.0005 −0.0001 0.0000]; and    -   H₅₀₄=[0.0000 0.0001 −0.0006 0.0018 −0.0032 0.0038 −0.0055        −0.0032 0.0131 −0.0233 0.0296 −0.0296 0.0229 −0.0180 0.0078        −0.0023 0.0002 0.0001 −0.0000]

Referring again to FIG. 9, the VoIP transcoder 500 receives an inputnarrowband signal 512. The input narrowband signal 512 may be, forexample, encoded using the G.711 protocol, or other narrowbandcommunications protocols. The input narrowband signal 512 is decoded bythe narrowband decoder 506 into a decoded signal 514 in accordance withthe relevant communications protocol.

The decoded signal 514 is input to both the first combined filter 502and the second combined filter 504, which filter the signal and producea lower sub-band signal and an upper sub-band signal, respectively. Thelower sub-band signal and the upper sub-band signal are then encoded bythe lower sub-band encoder 508 and the upper sub-band encoder 510,respectively. In one embodiment, the wideband communications protocolused is G.722 and the lower and upper sub-band encoders 508 and 510 areADPCM encoders operating in accordance with the G.722 protocol.

The encoders 508 and 510 output encoded lower and upper sub-band signalsthat are combined in the adder 518 to create a wideband output signal516.

Reference is now made to FIG. 11, which shows, in flowchart form, amethod 570 for transcoding a narrowband signal into a wideband signal ina VoIP system.

The method 570 begins in step 572 with the receiving of the inputnarrowband signal at a narrowband decoder. In step 574, the inputnarrowband signal is decoded by the narrowband decoder, which outputs adecoded signal. This decoded signal is then input to both a firstcombined filter and a second combined filter.

In step 576, the first combined filter filters the decoded signal toproduce a lower sub-band signal. Similarly, in step 578, the secondcombined filter filters the decoded signal to produce an upper sub-bandsignal. In step 580, the upper and lower sub-band signals are eachencoded by respective upper and lower sub-band encoders to produceencoded upper and lower sub-band signals. These encoded upper and lowersub-band signals are then combined and output as the wideband signal instep 582.

The filter-combination used to convert a narrowband signal into awideband signal may also be employed in mixing a narrowband signal and awideband signal to produce a wideband output signal.

Reference is now made to FIG. 12, which shows a block diagram of a VoIPmixer 600 for mixing an input narrowband signal with an input widebandsignal to produce an output wideband signal.

In a conventional hybrid mixer for mixing an input narrowband signalwith an input wideband signal to produce an output wideband signal, theinput signals are decoded in the conventional manner and the decodednarrowband signal is then up-sampled and low pass filtered before beingmixed with the decoded wideband signal. The mixed signal is then passedthrough a transmit QMF and encoded in the conventional manner. Thedecoding of the input wideband signal includes separating the upper andlower sub-bands, decoding each sub-band, and passing the decodedsub-bands through a receive QMF to produce a decoded wideband signal formixing.

Using filter-combination and sub-band mixing, the VoIP mixer 600eliminates the need for receive and transmit QMFs. The VoIP mixer 600includes a narrowband decoder 602, a lower combined filter 604, an uppercombined filter 606, a lower sub-band decoder 608, an upper sub-banddecoder 610, a first mixer 612, a second mixer 614, a lower sub-bandencoder 616, an upper sub-band encoder 618, and an adder 638. The VoIPmixer 600 receives an input narrowband signal 620 and an input widebandsignal 622 and outputs a mixed wideband signal 624.

The narrowband decoder 602 decodes the input narrowband signal 620 intoa decoded narrowband signal, in accordance with the relevant narrowbandcommunications protocol, which, in one embodiment, is G.711. The decodednarrowband signal is then applied to both the lower and upper combinedfilters 604, 606, as described above with reference to the VoIPtranscoder 500 (FIG. 9) and its first and second combined filters 502,504 (FIG. 9). The lower and upper combined filters 604, 606 output afirst lower sub-band signal 626 and a first upper sub-band signal 628,respectively.

The input wideband signal 622 is split into its encoded lower and uppersub-band signals, which are input to the lower and upper sub-banddecoders 608, 610, respectively. The decoders 608, 610 decode thesub-band signals in accordance with the relevant wideband communicationsprotocol, which, in one embodiment, is G.722. The lower and upperdecoders 608, 610 output a second lower sub-band signal 630 and a secondupper sub-band signal 632.

The first mixer 612 mixes the first lower sub-band signal 626 and thesecond lower sub-band signal 630 to produce a mixed lower sub-bandsignal 634. The second mixer 614 mixes the first upper sub-band signal628 and the second upper sub-band signal 632 to produce a mixed uppersub-band signal 636.

The two mixed signals 634, 636 are encoded by the lower and uppersub-band encoders 616, 618, respectively, in accordance with therelevant wideband communications protocol, which, in one embodiment, isG.722. Following the encoders 616, 618, the encoded mixed signals arecombined in the adder 638 to create the mixed wideband signal 624.

Reference is now made to FIG. 13, which shows, in flowchart form, amethod 650 for mixing an input narrowband signal with an input widebandsignal to produce an output wideband signal, in a VoIP system.

The method 650 begins in step 652 when the input narrowband signal andthe input wideband signal are received. In step 654, the inputnarrowband signal is decoded in accordance with the relevant narrowbandcommunications protocol, thereby producing a decoded narrowband signal.In step 656, the input wideband signal is separated into its upper andlower sub-band signals and the sub-band signals are decoded inaccordance with the relevant wideband communications protocol.

In step 658, the decoded narrowband signal is input to a lower combinedfilter and to an upper combined filter, which produce a first lowersub-band signal and a first upper sub-band signal, respectively. Thedecoding operation in step 656 results in a second lower sub-band signaland a second upper sub-band signal.

The first and second lower sub-band signals are mixed together in afirst mixer in step 660 and the first and second upper sub-band signalsare mixed together in a second mixer in step 662. The first and secondmixers output lower and upper mixed signals, respectively.

In step 664, the lower and upper mixed signals are encoded using a lowerand an upper sub-band encoder, respectively, in accordance with therelevant communications protocol. The encoded lower and upper sub-bandsignals are then combined and output as the output wideband signal instep 666.

The present invention may be embodied in other specific forms withoutdeparting from the spirit or essential characteristics thereof. Certainadaptations and modifications of the invention will be obvious to thoseskilled in the art. Therefore, the above discussed embodiments areconsidered to be illustrative and not restrictive, the scope of theinvention being indicated by the appended claims rather than theforegoing description, and all changes which come within the meaning andrange of equivalency of the claims are therefore intended to be embracedtherein.

1. A VoIP mixer for mixing a first input signal with a second inputsignal, the first and second input signals being signals having encodedtherein a first and second correlation parameter, respectively, the VoIPmixer comprising: (a) a first decoder for receiving the first inputsignal and generating a first decoded signal, the first decoderextracting the first correlation parameter from the first input signal;(b) a second decoder for receiving the second input signal andgenerating a second decoded signal, the second decoder extracting thesecond correlation parameter from the second input signal; (c) a mixercoupled to the first and second decoders, the mixer receiving the firstand second decoded signals and producing a mixed signal; (d) a parameterestimator coupled to the first and second decoders, the parameterestimator receiving the first and second correlation parameters andoutputting an open loop parameter estimate; and (e) an encoder coupledto the mixer and the parameter estimator, the encoder receiving themixed signal and the open loop parameter estimate and outputting anencoded signal, wherein the encoder includes a closed-loop analyzer forcreating the encoded signal and wherein the closed-loop analyzer employsthe open loop parameter estimate; wherein the correlation parametersinclude pitch data, the parameter estimator includes a pitch estimator,and the open loop parameter includes an open loop pitch estimate.
 2. TheVoIP mixer claimed in claim 1, wherein the closed loop analyzer includesa closed loop pitch analyzer and the closed loop pitch analyzer employsthe open loop pitch estimate as a starting point for closed loop pitchanalysis.
 3. The VoIP mixer claimed in claim 1, wherein the pitchestimator selects the open loop pitch estimate based upon the inputsignal having strongest pitch energy, as defined by the pitch data. 4.The VoIP mixer claimed in claim 1, wherein the first and second inputsignals are encoded according to a communications protocol selected fromthe group comprising G.729, G.729(A), and G.722.2.
 5. A method formixing a first input signal with a second input signal in a VoIP system,the first and second input signals comprising signals having encodedtherein first and second correlation parameters, respectively, themethod comprising the steps of: (a) in a decoder, extracting the firstand second correlation parameters from the first and second inputsignals, decoding the first and second input signals and outputting afirst decoded signal and a second decoded signal; (b) mixing the firstand second decoded signals to produce a mixed signal; (c) determining anopen loop parameter estimate based upon the extracted first and secondcorrelation parameters; and (d) encoding the mixed signal, wherein thestep of encoding includes performing a closed loop analysis to obtain amixed signal correlation parameter for use in encoding the mixed signal,and wherein the closed loop analysis employs the open loop parameterestimate; wherein the correlation parameters include pitch data, and thestep of determining includes determining an open loop pitch estimatebased upon the pitch data.
 6. The method claimed in claim 5, wherein thestep of performing closed loop analysis employs the open loop pitchestimate as a starting point for closed loop pitch analysis.
 7. Themethod claimed in claim 5, wherein the step of determining includesselecting the open loop pitch estimate based upon the input signalhaving strongest pitch energy, as defined by the pitch data.
 8. Themethod claimed in claim 5, wherein the first and second input signal areencoded according to a communications protocol selected from the groupcomprising G.729, G.729(A), and G.722.2.